Bipolar transistor with graded base layer

ABSTRACT

A semiconductor material which has a high carbon dopant concentration includes gallium, indium, arsenic and nitrogen. The disclosed semiconductor materials have a low sheet resistivity because of the high carbon dopant concentrations obtained. The material can be the base layer of gallium arsenide-based heterojunction bipolar transistors and can be lattice-matched to gallium arsenide emitter and/or collector layers by controlling concentrations of indium and nitrogen in the base layer. The base layer can have a graded band gap that is formed by changing the flow rates during deposition of III and V additive elements employed to reduce band gap relative to different III-V elements that represent the bulk of the layer. The flow rates of the III and V additive elements maintain an essentially constant doping-mobility product value during deposition and can be regulated to obtain pre-selected base-emitter voltages at junctions within a resulting transistor.

RELATED APPLICATION

[0001] This application is a continuation-in-part of of U.S. applicationSer. No. 09/995,079 filed on Nov. 27, 2001; which claims the benefit ofU.S. Provisional Application No. 60/253,159, filed Nov. 27, 2000 theteachings of both which are incorporated herein in their entirety. Thisapplication also claims the benefit of U.S. Provisional Applicationfiled Apr. 5, 2002, having Attorney's Docket No. 0717.2013-005, andentitled “Heterojunction Bipolar Transistor with Graded Base”; and ofU.S. Provisional Application filed Apr. 10, 2002, having Attorney'sDocket No. 0717.2013-006, and entitled, “Bipolar Transistor with GradedBase Layer,” the teachings of all of which are incorporated herein intheir entirety.

BACKGROUND OF THE INVENTION

[0002] Bipolar junction transistors (BJT) and heterojunction bipolartransistor (HBT) integrated circuits (ICs) have developed into animportant technology for a variety of applications, particularly aspower amplifiers for wireless handsets, microwave instrumentation, andhigh speed (>10 Gbit/s) circuits for fiber optic communication systems.Future needs are expected to require devices with lower voltageoperation, higher frequency performance, higher power added efficiency,and lower cost production. The turn-on voltage (V_(be,on)) of a BJT orHBT is defined as the base-emitter voltage (V_(be)) required to achievea certain fixed collector current density (J_(c)). The turn-on voltagecan limit the usefulness of devices for low power applications in whichsupply voltages are constrained by battery technology and the powerrequirements of other components.

[0003] Unlike BJTs in which the emitter, base and collector arefabricated from one semiconductor material, HBTs are fabricated from twodissimilar semiconductor materials in which the emitter semiconductormaterial has a large band gap (also referred to as “energy gap”) thanthe semiconductor material from which the base is fabricated. Thisresults in a superior injection efficiency of carriers from the base tocollector over BJTs because there is a built in barrier impeding carrierinjection from the base back to the emitter. Selecting a base with asmaller band gap decreases the turn-on voltage because an increase inthe injection efficiency of carriers from the base into the collectorincreases the collector current density at a given base-emitter voltage.

[0004] HBTs, however, can suffer from the disadvantage of having anabrupt discontinuity in the band alignment of the semiconductor materialat the heterojunction can lead to a conduction band spike at theemitter-base interface of the HBT. The effect of this conduction bandspike is to block electron transport out of the base into the collector.Thus, electron stay in the base longer resulting in an increased levelof recombination and a reduction of collector current gain (β_(dc)).Since, as discussed above, the turn-on voltage of heterojunction bipolartransistors is defined as the base-emitter voltage required to achieve acertain fixed collector current density, reducing the collector currentgain effectively raise the turn-on voltage of the HBT. Consequently,further improvements in the fabrication of semiconductor materials ofHBTs are necessary to lower the turn-on voltage, and thereby improve lowvoltage operation devices.

SUMMARY OF THE INVENTION

[0005] The present invention provides an HBT having an n-dopedcollector, a base formed over the collector and composed of a III-Vmaterial that includes indium and nitrogen, and an n-doped emitterformed over the base. The III-V material of the base layer has a carbondopant concentration of about 1.5×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³. In apreferred embodiment, the base layer includes the elements gallium,indium, arsenic, and nitrogen. The presence of indium and nitrogenreduces the band gap of the material relative to the band gap of GaAs.In addition, the dopant concentration in the material is high, the sheetresistivity (R_(sb)) is low. These factors result in a lower turn-onvoltage relative to HBTs having a GaAs base layer with a similar dopantconcentration.

[0006] In a preferred embodiment, the III-V compound material system canbe represented by the formula Ga_(1-x)In_(x)As_(1-y)N_(y). It is knownthat the energy-gap of Ga_(1-x)In_(x)As drops substantially when a smallamount of nitrogen is incorporated into the material. Moreover, becausenitrogen pushes the lattice constant in the opposite direction fromindium, Ga_(1-x)In_(x)As_(1-y)N_(y), alloys can be grown lattice-matchedto GaAs by adding the appropriate ratio of indium to nitrogen to thematerial. Thus, excess strain which results in an increased band gap andmisfit dislocation of the material can be eliminated. The ratio ofindium to nitrogen is thus selected to reduce or eliminate strain. In apreferred embodiment of the present invention, x=3y in theGa_(1-x)In_(x)As_(1-y)N_(y) base layer of the HBT.

[0007] In conventional HBTs having a GaAs, the current gain typicallydecreases with increasing temperature as a result of higher injection ofholes to the emitter, higher space charge layer recombination current,and possible shorter diffusion length in the base. In HBTs having aGaInAsN base layer, a significant increase in current gain is found withincreasing temperature (approximately 0.3% for each 1° C. rise). Thisresult is interpreted as an increase in diffusion length with increasingtemperature. Such an effect is expected if electrons at the bottom ofthe band are confined in states that are at least partially localized,and with increasing temperature they are thermally excited out of thosestates to others in which the electrons can diffuse more readily. Thus,engineering the base layer with GaInAsN improves temperaturecharacteristics in HBTs of the invention and reduces the need fortemperature compensation sub-circuitry.

[0008] HBTs having a GaInAsN base layer have improved common emitteroutput characteristics over conventional HBTs having a GaAs base layer.For example, HBTs having GaInAsN base layers have lower offset and kneevoltages than conventional HBTs having a GaAs base layer.

[0009] In one embodiment, the transistor is a double heterojunctionbipolar transistor (DHBT) having a base composed of a semiconductormaterial which is different from the semiconductor material from whichthe emitter and collector are fabricated. In a preferred embodiment of aDHBT, the Ga_(1-x)In_(x)As_(1-y)N_(y) base layer can be represented bythe formula Ga_(1-x)In_(x)As_(1-y)N_(y), the collector is GaAs and theemitter is selected from InGaP, AlInGaP and AlGaAs.

[0010] Another preferred embodiment of the invention relates to a HBT orDHBT in which the height of the conduction band spike is lowered incombination with lowering of the base layer energy gap (E_(gb)).Conduction band spikes are caused by a discontinuity in the conductionband at the base/emitter heterojunction or the base/collectorheterojunction. Reducing the lattice strain by lattice matching the baselayer to the emitter and/or the collector layer reduces the conductionband spike. This is typically done by controlling the concentration ofthe nitrogen and the induim in the base layer. Preferably, the baselayer has the formula Ga_(1-x)In_(x)As_(1-y)N_(y) wherein x is aboutequal to 3y.

[0011] In one embodiment, the base can be compositionally graded toproduce a graded band gap layer having a smaller band gap at thecollector and a larger band gap at the emitter. Preferably, the baselayer band gap is about 20 meV to about 120 meV lower at a surface ofthe base layer in contact with the collector than at a surface of thebase layer in contact with the emitter. More preferably, the band gap ofthe base layer varies linearly across the base layer from the collectorto the emitter.

[0012] Addition of nitrogen and indium to a GaAs semiconductor materiallowers the band gap of the material. Thus, Ga_(1-x)In_(x)As_(1-y)N_(y)semiconductor materials have a lower band gap than that of GaAs. Incompositionally graded Ga_(1-x)In_(x)As_(1-y)N_(y) base layers of theinvention, the reduction in band gap of the base layer is larger at thecollector than at the emitter. However, the average band gap reductionin comparison to the band gap GaAs across the base layer is, typically,about 10 meV to about 300 meV. In one embodiment, the average band gapreduction in comparison to the band gap GaAs across the base layer is,typically, about 80 meV to about 300 meV. In another embodiment, theaverage band gap reduction in comparison to the band gap GaAs across thebase layer is, typically, about 10 meV to about 200 meV. This reducedband gap results in a lower turn-on voltage (V_(be,on)) for HBTs havinga compositionally graded Ga_(1-x)In_(x)As_(1-y)N_(y) base layer than forHBTs having a GaAs base layer because the principal determinant inV_(be,on) is the intrinsic carrier concentration in the base. Theintrinsic carrier concentration (n_(i)) is calculated from the followingformula:

n _(i) =N _(c) N _(v) exp(−E _(g/kT))

[0013] In the above formula, N_(c) is the effective density ofconduction band states; N_(v) is the effective density of valence bandstates; E_(g) is the band gap; T is the temperature; and is Boltzmannconstant. As can be seen from the formula, the intrinsic carrierconcentration in the base is largely controlled by the band gap of thematerial used in the base.

[0014] Grading the band gap of the base layer from a larger band gap atthe base-emitter interface to a smaller band gap at the base-collectorinterface introduces a quasielectric field, which accelerates electronsacross the base layer in npn bipolar transistors. The electric fieldincreases the electron velocity in the base, decreasing the base transittime which improves the RF (radiofrequency) performance and increasesthe collector current gain (also called dc current gain). The dc gain(β_(dc)), in the case of HBTs with heavily doped base layers, is limitedby bulk recombination in the neutral base (n=1). The dc current gain canbe estimated formula 1:

β_(dc) ≈ντ/w _(b)  (1)

[0015] In formula (1), ν is the average minority carrier velocity in thebase; τ is the minority carrier lifetime in the base; and w_(b) is thebase thickness. Properly grading the base layer in HBT having a GaInAsNbase layer results in a significant increase in β_(dc) in comparison toa non-graded GaInAsN base layer due to the increased electron velocity.

[0016] To achieve a band gap that is graded over the thickness of thebase layer, the base layer is prepared such that it has a higherconcentration of indium and/or nitrogen at a first surface of the baselayer, near the collector, than at a second surface of the base layernearer the emitter. The change in the indium and/or nitrogen contentpreferably changes linearly across the base layer resulting in alinearly graded band gap. Preferably, the concentration of dopant (e.g.,carbon) remains constant throughout the base layer. In one embodiment, aGa_(1-x)In_(x)As_(1-y)N_(y) base layer, for example a base layer of aDHBT, is graded such that x and 3y are about equal to 0.01 at thecollector and are graded to about zero at the emitter. In anotherembodiment, the Ga_(1-x)In_(x)As_(1-y)N_(y) base layer is graded from avalue of x in the range of about 0.2 to about 0.02 at a surface of thebase layer in contact with the collector to a value of x in the range ofabout 0.1 to zero at a surface of the base layer in contact with theemitter, provided that the value of x is larger at the surface of thebase layer in contact with the collector than at the surface of the baselayer in contact with the emitter. In this embodiment, y can remainconstant throughout the base layer or can be linearly graded. When y islinearly graded, the base layer is graded from a value of y in the rangeof about 0.2 to about 0.02 at a surface of the base layer in contactwith the collector to a value of y in the range of about 0.1 to zero ata surface of the base layer in contact with the emitter, provided thatthe value of y is larger at the surface of the base layer in contactwith the collector than at the surface of the base layer in contact withthe emitter. In a preferred embodiment, x is about 0.006 at thecollector and is linearly graded to about 0.01 at the emitter. In a morepreferred embodiment, x is about 0.006 at the collector and is linearlygraded to about 0.01 at the emitter, and y is about 0.001 throughout thebase layer.

[0017] In another embodiment, the invention is a method of forming agraded semiconductor layer having an essentially linear grade of bandgap and an essentially constant doping-mobility product from a firstsurface through the layer to a second surface. The method includes:

[0018] a) comparing the doping-mobility product of calibration layers,each of which is formed at distinct flow rates of one of either anorganometallic compound depositing aatom from Group III or V of thePeriodic Table, or of a carbon tetrahalide compound depositing carbon,whereby the relative organometallic compound and carbon tetrahalide flowrates required to form an essentially constant doping-mobility productare determined; and

[0019] b) flowing the organometallic and carbon tetrahalide compoundsover a surface at said relative rates to form an essentially constantdoping-mobility product, said flow rates changing during deposition tothereby form an essentially linear grade of band gap through the gradedsemiconductor layer.

[0020] The base layer can also be dopant-graded such that the dopantconcentration is higher near the collector and decrease gradually acrossthe thickness of the base to the base emitter heterojunction.

[0021] Another method of minimizing the conduction band spike is toinclude one or more transitional layers at the heterojunction.Transitional layers having low band gap set back layers, graded band gaplayers, doping spikes or a combination of thereof can be employed tominimize the conduction band spike. In addition, one or morelattice-matched layers can be present between the base and emitter orbase and collector to reduce the lattice strain on the materials at theheterojunction.

[0022] The present invention also provides a method of fabricating anHBT and a DHBT. The method includes growing a base layer composed ofgallium, indium, arsenic and nitrogen over an n-doped GaAs collector.The base layer can be grown employing internal and/or external carbonsources to provide a carbon-doped base layer. An n-doped emitter layeris then grown over the base layer. The use of an internal and externalcarbon source to provide the carbon dopant for the base layer can helpform a material with a relatively high carbon dopant concentration.Typically, dopant levels of about 1.5×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm ⁻³are achieved using the method of the invention. In a preferredembodiment, dopant levels of about 3.0×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³can be achieved with the method of the invention. A higher dopantconcentration in a material reduces the sheet resistivity and band gapof the material. Thus, the higher the dopant concentration in the baselayer of an HBT and DHBT, the lower the turn-on voltage of the device.

[0023] The present invention also provides a material represented by theformula Ga_(1-x)In_(x)As_(1-y)N_(y) in which x and y are each,independently, about 1.0×10⁻⁴ to about 2.0×10⁻¹. Preferably, x is aboutequal to 3y. More preferably, x and 3y are about equal to 0.01. In oneembodiment, the material is doped with carbon at a concentration ofabout 1.5×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³. In a specific embodiment,the carbon dopant concentration is about 3.0×10¹⁹ cm⁻³ to about 7.0×10¹⁹cm⁻³.

[0024] The reduction in turn-on voltage can result in better managementof the voltage budget on both wired and wireless GaAs-based RF circuits,which are constrained either by standard fixed voltage supplies or bybattery output. Lowering the turn-on voltage can also alter the relativemagnitude of the various base current components in a GaAs-based HBT. DCcurrent gain stability as a function of both junction temperature andapplied stress has been previously shown to rely critically on therelative magnitudes of the base current components. A reduction inreverse hole injection enabled by a low turn-on voltage is favorable forboth the temperature stability and long-term reliability of the device.Thus, relatively strain-free Ga_(1-x)In_(x)As_(1-y)N_(y) base materialshaving a high dopant concentration can significantly enhance RFperformance in GaAs-based HBTs and DHBTs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 illustrates a InGaP/GaInAsN DHBT structure of a preferredembodiment of the invention in which x is about equal to 3y.

[0026]FIG. 2 is a Gummel plot which graphical illustrates the base andcollector currents as a function of turn on voltage for an InGaP/GaInAsNDHBT of the invention and for an InGaP/GaAs HBT and a GaAs/GaAs BJT ofthe prior art.

[0027]FIG. 3 is a graphical illustration of turn on voltage (atJ_(c)=1.78 A/cm²) as a function of base sheet resistance for anInGaP/GaInAsN DHBT of the invention and for an InGaP/GaAs HBT and aGaAs/GaAs BJT of the prior art.

[0028]FIG. 4 illustrates the photoluminescence spectra measured at 77°K. of an InGaP/GaInAsN DHBT of the invention and of an InGaP/GaAs HBT ofthe prior art, both with a nominal base thickness of 1000 Å.Photoluminescence measurements were taken after etching off the InGaAsand GaAs cap layers, selectively stopping at the top of the InGaPemitter. The band gap of the n-type GaAs collector of both theInGaP/GaAs HBT and the InGaP/GaInAsN DHBT was 1.507 eV. The band gap ofthe p-type GaAs base layer of the InGaP/GaAs HBT was 1.455 eV, whereasthe band gap of the p-type GaInAsN base layer of the InGaP/GaInAsN was1.408 eV.

[0029]FIG. 5 illustrates double crystal x-ray diffraction (DCXRD)spectra of a InGaP/GaInAsN DHBT of the invention and a InGaP/GaAs HBT ofthe prior art, both having a nominal base thickness of 1500 Å. Thepositions of the base layers peaks are marked.

[0030]FIG. 6 is a Polaron C-V profile which illustrates the carrierconcentration across the thickness of the base layer in an InGaP/GaInAsNDHBT of the invention and an InGaP/GaAs HBT of the prior art. Both theInGaP/GaInAsN DHBT and an InGaP/GaAs HBT have a nominal base thicknessof 1000 Å. Both Polaron profiles are obtained after selectively etchingdown to the top of the base layer.

[0031]FIG. 7a illustrates a preferred InGaP/GaInAsN DHBT structure whichhas a transitional layer between the emitter and the base and atransitional layer and lattice matched layer between the collector andthe base.

[0032]FIGS. 7b and 7 c illustrates a alternative InGaP/GaInAsN DHBTstructure having compositionally graded base layers.

[0033]FIG. 8 is a graph of the doping*mobility produce as a function ofcarbon tetrabromide flow rate in a carbon doped GaInAsN base layersgrown at a constant indium source gas flow rate (“TMIF” is the trimethylindium flow rate).

[0034]FIG. 9 is a graph of the TMIF versus the carbon tetrabromide flowrate needed to obtain a constant doping*mobility product while growing acarbon doped compositionally graded GaInAsN base layers.

[0035]FIG. 10 is a graph showing that InGaP/GaInAsN HBTs have a lowerturn-on voltage than InGaP/GaAs HBTs.

[0036]FIG. 11 is a graph of the ΔV_(bc) versus carbon tetrabromide flowrate of carbon doped GaInAsN base layers grown at a constant TMIF.

[0037]FIG. 12 is a graph of the ΔV_(be) versus TMIF.

[0038]FIG. 13 is the structure of DHBTs having compositionally gradedbase layers used in the experiments in Example 2.

[0039]FIG. 14 is the structure of DHBTs having constant composition baselayers used in the experiments in Example 2.

[0040]FIG. 15 is a Gummel plot comparing DHBTs having a constantcomposition GaInAsN base layer to DHBTs having a compositionally gradeGaInAsN base layer.

[0041]FIG. 16 is a graph of the DC current gain as a function of basesheet resistance for DHBTs having a constant composition GaInAsN baselayer to DHBTs having a compositionally grade GaInAsN base layer.

[0042]FIG. 17 is a Gummel plot conparing a DHBT having a compositionallygrade GaInAsN base layer to two DHBTs having a constant compositionGaInAsN base layer.

[0043]FIG. 18 is a graph of comparing DC current gain as a function ofcollector current density for a DHBT having a compositionally gradeGaInAsN base layer to two DHBTs having a constant composition GaInAsNbase layer.

[0044]FIG. 19 is a graph comparing the extrapolated current gain cutofffrequency as a function of collector current density of DHBTs having aconstant composition GaInAsN base layer to DHBTs having acompositionally grade GaInAsN base layer.

[0045]FIG. 20 is a graph comparing the small signal current gain as afunction of frequency of DHBTs having a constant composition GaInAsNbase layer to DHBTs having a compositionally grade GaInAsN base layer.

[0046]FIG. 21 is a graph of the peak f_(t) as a function of BV_(ceo) ofconstant composition GaInAsN base layer and DHBTs having acompositionally grade GaInAsN base layer to conventional HBTs having aGaAs base layer.

DETAILED DESCRIPTION OF THE INVENTION

[0047] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0048] A III-V material is a semiconductor having a lattice comprisingat least one element from Column III(A) of the Periodic Table and atleast one element from column V(A) of the Periodic Table. In oneembodiment, the III-V material is a lattice comprised of gallium,indium, arsenic and nitrogen. Preferably, the III-V material can berepresented by the formula Ga_(1-x)In_(x)As_(1-y)N_(y) wherein x and yare each, independently, about 1.0×10⁻⁴ to about 2.0×10⁻¹. Morepreferably, x is about equal to 3y. In a most preferred embodiment, xand 3y are about 0.01.

[0049] The term “transitional layer,” as used herein, refers to a layerthat is between the base/emitter heterojunction or the base/collectorheterojunction and has the function of minimizing the conduction bandspike of the heterojunction. One method of minimizing the conductionband spike is to use a series of transitional layers wherein the bandgaps of the transitional layers gradually decrease from the transitionallayer nearest in proximity to the collector to the transitional layernearest in proximity to the base in a base/collector heterojunction.Likewise, in a emitter/base heterojunction, the band gaps of thetransitional layers gradually decrease from the transitional layernearest in proximity to the emitter to the transitional layer nearest inproximity to the base. Another method of minimizing the conduction bandspike is to use a transitional layer having a graded band gap. The bandgap of a transitional layer can be graded by grading the dopantconcentration of the layer. For example, the dopant concentration of thetransitional layer can be higher near the base layer and can begradually decreased near the collector or the emitter. Alternatively,lattice strain can be used to provide a transitional layer having agraded band gap. For example, the transitional layer can becompositionally graded to minimize the lattice strain at the surface ofthe layer in contact with the base and increase the lattice strain atthe surface in contact with the collector or emitter. Another method ofminimizing the conduction band spike is to use a transitional layerhaving a spike in the dopant concentration. One or more of theabove-described methods for minimizing the conduction band spike can beused in the HBTs of the invention. Suitable transitional layers for theHBTs of the invention include GaAs, InGaAs and InGaAsN.

[0050] A lattice-matched layer is a layer which is grown on a materialhaving a different lattice constant. The lattice-matched layer typicallyhas a thickness of about 500 Å or less and essentially conforms to thelattice constant of the underlying layer. This results in a band gapintermediate between the band gap of the underlying layer and the bandgap of the lattice-matched material if it were not strained. Methods offorming lattice-matched layers are known to those skilled in the art andcan be found in on pages 303-328 of Ferry, et al., Gallium ArsenideTechnology (1985), Howard W. Sams & Co., Inc. Indianapolis, Ind. theteachings of which are incorporated herein by reference. An example of asuitable material for lattice-matched layers of the HBTs of theinvention is InGaP.

[0051] HBTs and DHBTs with Constant-Composition Base Layers

[0052] The HBTs and DHBTs of the invention can be prepared using asuitable metalorganic chemical vapor deposition (MOCVD) epitaxial growthsystem. Examples of suitable MOCVD epitaxial growth systems are AIXTRON2400 and AIXTRON 2600 platforms. In the HBTs and the DHBTs prepared bythe method of the invention, typically, an un-doped GaAs buffer layercan be grown after in-situ oxide desorption. For example, a subcollectorlayer containing a high concentration of an n-dopant (e.g., dopantconcentration about 1×10¹⁸ cm⁻³ to about 9×10¹⁸ cm⁻³) can be grown at atemperature of about 700° C. A collector layer with a low concentrationof a n-dopant (e.g., dopant concentration about 5×10¹⁵ cm⁻³ to about5×10¹⁶ cm⁻³) can be grown over the subcollector at a temperature ofabout 700° C. Preferably, the subcollector and the collector are GaAs.The subcollector layer typically has a thickness of about 4000 Å toabout 6000 Å, and the collector typically has a thickness of about 3000Å to about 5000 Å. In one embodiment, the dopant in the subcollectorand/or the collector is silicon.

[0053] Optionally, a lattice-match InGaP tunnel layer can be grown overthe collector under typical growth conditions. A lattice-matched layergenerally has a thickness of about 500 Å or less, preferably about 200 Åor less, and has a dopant concentration of about 1×10¹⁶ cm⁻³ to about1×10¹⁸ cm⁻³.

[0054] One or more transitional layers can optionally be grown undertypical growth conditions on the lattice-matched layer or on thecollector if no lattice-match layer is used. Transitional layer can beprepared from n-doped GaAs, n-doped InGaAs or n-doped InGaAsN.Transitional layers optionally can be compositionally or dopant gradedor can contain a dopant spike. Transitional layers typically have athickness of about 75 Å to about 25 Å. The carbon doped GaInAsN baselayer was grown over the collector if neither a lattice-matched or atransitional layer was used.

[0055] The base layer is grown at a temperature below about 750° C. andtypically is about 400 Å to about 1500 Å thick. In a preferredembodiment, the base layer is grown at a temperature of about 500° C. toabout 600° C. Optionally, the carbon doped GaInAsN base layer can begrown over the transitional layer or over the lattice-matched layer if atransitional layer is not used. The base layer can be grown using asuitable gallium source, such as trimethylgallium or triethylgallium, anarsenic source, such as arsine, tributylarsine or trimethylarsine, anindium source, such as trimethylindium, and a nitrogen source, such asammonia or dimethylhydrazine. A low molar ratio of the arsenic source tothe gallium source is preferred. Typically, the molar ratio of thearsenic source to the gallium source is less than about 3.5. Morepreferably, the ratio is about 2.0 to about 3.0. The levels of thenitrogen and indium sources are adjusted to obtain a material which wascomposed of about 0.01% to about 20% indium and about 0.01% to about 20%nitrogen. In a preferred embodiment, the indium content of the baselayer is about three times higher than the nitrogen content. In a morepreferred embodiment, the indium content is about 1% and the nitrogencontent is about 0.3%. In the present invention, a GaInAsN layer havinga high carbon dopant concentration of about 1.5×10¹⁹ cm⁻³ to about7.0×10¹⁹ cm⁻³ can be obtained by using an external carbon sourceorganometallic source, specifically, the gallium source. An example of asuitable external carbon source is carbon tetrabromide. Carbontetrachloride is also an effective external carbon source.

[0056] Optionally, one or more transitional layers can be grown ofn-doped GaAs, n-doped InGaAs or n-doped InGaAsN between the base and theemitter. Transitional layers between the base and emitter are relativelylightly doped (e.g., about 5.0×10¹⁵ cm⁻³ to about 5.0×10¹⁶ cm⁻³) andoptionally contain a dopant spike. Preferably, transitional layers areabout 25 Å to about 75 Åthick.

[0057] An emitter layer is grown over the base, or optionally over atransitional layer, at a temperature of about 700° C. and is typicallyabout 400 Å to about 1500 Å thick. The emitter layer includes, forexample, InGaP, AlInGaP, or AlGaAs. In a preferred embodiment, theemitter layer includes InGaP. The emitter layer can be n-doped at aconcentration of about 1.0×10¹⁷ cm⁻³ to about 9.0×10¹⁷ cm⁻³. Anemitter-contact layer that includes GaAs containing a high concentrationof an n-dopant (e.g., about 1.0×10¹⁸ cm⁻³ to about 9×10¹⁸ cm⁻³)optionally is grown over the emitter at a temperature of about 700° C.Typically, the emitter contact layer is about 1000 Å to about 2000 Åthick.

[0058] A InGaAs layer with a ramped-in indium composition and a highconcentration of an n-dopant (e.g., about 5×10¹⁸ cm⁻³ to about 5×10¹⁹cm⁻³) is grown over the emitter contact layer. This layer typically isabout 400 Å to about 1000 Å thick.

EXAMPLE 1

[0059] To illustrate the effect of reducing the band gap of the baselayer and/or minimizing the conduction band spike at the emitter/baseheterojunction, three different types of GaAs-based bipolar transistorstructures were compared: GaAs emitter/GaAs base BJTs, InGaP/GaAs HBTs,and InGaP/GaInAsN DHBTs of the invention. A general representation ofInGaP/GaInAsN DHBT structures used in the following experiments isillustrated in FIG. 1. There is only one heterojunction at theemitter/base interface since the base and the collector are both formedfrom GaAs. The GaAs base layer of the InGaP/GaAs HBT has a larger bandgap than the base of the InGaP/GaInAsN DHBT. GaAs/GaAs BJTs have noheterojunctions since the emitter, collector and base are all made ofGaAs. Thus, GaAs BJT structures are used as a reference to determinewhat impact, if any, a conduction band spike at the base-emitterinterface has on the collector current characteristics of InGaP/GaAsHBTs. In the DHBTs of FIG. 1, InGaP is chosen as the emitter materialwith the Ga_(1-x)In_(x)As_(1-y)N_(y) base because InGaP has a wide bandgap, and its conduction band lines up with the conduction band of theGa_(1-x)In_(x)As_(1-y)N_(y) base. Comparison of the InGaP/GaInAsN DHBTsof FIG. 1 and the InGaP. GaAs HBTs are used to determine the effect oncollector current density of having a base layer with a lower band gap.

[0060] All of the GaAs devices used in the following discussion haveMOCVD-grown, carbon-doped base layers in which the dopant concentrationvaried from about 1.5×10¹⁹ cm⁻³ to about 6.5×10¹⁹ cm⁻³ and a thicknessvaried from about 500 Å to about 1500 Å, resulting in a base sheetresistivity (R_(sb)) of between 100 Ω/□ and 400 Ω/□. Large area devices(L=75 μm×75 μm) were fabricated using a simple wet-etching process andtested in the common base configuration. Relatively small amounts ofindium (x˜1%) and nitrogen (y˜0.3%) were added incrementally to form twoseparate sets of InGaP/GaInAsN DHBTs. For each set, growth has beenoptimized to maintain high, uniform carbon dopant levels (>2.5×10¹⁹cm⁻³), good mobility (˜85 cm²/V-s), and high dc current gain (>60 atR_(sb) ˜300 Ω/□).

[0061] Typical Gummel plots from a GaAs/GaAs BJT, an InGaP/GaAs HBT andan InGaP/GaInAsN DHBT with comparable base sheet resistivities wereplotted and overlaid in FIG. 2. The collector currents of the InGaP/GaAsHBT and GaAs/GaAs BJT were indistinguishable for over five orders ofmagnitude (decades) of current until differences in effective seriesresistance impacted the current-voltage characteristics. On the otherhand, the collector current of an InGaP/GaInAsN DHBT was two-fold higherthan the collector current of the GaAs/GaAs BJT and the InGaP/GaAs HBTover a wide bias range, corresponding to a 25.0 mV reduction in turn-onvoltage at a collector current density (J_(c)) of 1.78 A/cm². Theobserved increase in the low-bias base current (n=2 component) in theBJT is consistent with an energy-gap driven increase in space chargerecombination. The neutral base recombination component of the basecurrent in the InGaP/GaInAsN DHBT was driven higher than in theInGaP/GaAs HBT because of the increase in collector current, as well asreduction in the minority carrier lifetime or an increase in the carriervelocity (I_(nbr)=I_(c)w_(b)/vr). InGaP/GaInAsN DHBT devices preparedto-date have achieved a peak dc current gain of 68 for a device having abase sheet resistivity of 234 Ω/□ corresponding to a decrease in turn-onvoltage of 11.5 mV, and a peak dc current gain of 66 for a device havinga base sheet resistivity of 303 Ω/□, corresponding to a decrease inturn-on voltage of 25.0 mV. This represents the highest knowngain-to-base-sheet-resistance ratios (β/R_(sb)˜0.2−0.3) for these typesof structures. The energy-gap reduction in theGa_(1-x)In_(x)As_(1-y)N_(y) base, is responsible for the observeddecrease in turn-on voltage, as demonstrated by low temperature (77° K.)photoluminescence. DCXRD measurements indicate the lattice mismatch ofthe base layer is minimal (<250 arcsec).

[0062] In the diffusive limit, the ideal collector current density of abipolar transistor as a function of base-emitter voltage (V_(be)) can beapproximated as:

J _(c)=(qD _(n) n ² _(ib) /p _(b) w _(b))exp(qV _(be) /kT)  (2)

[0063] where

[0064] p_(b) and w_(b) base doping and width;

[0065] D_(n) diffusion coefficient;

[0066] n_(ib) intrinsic carrier concentration in the base.

[0067] By expressing n_(ib) as a function of base layer energy-gap(E_(gb)) and rewriting the product of base doping and thickness in termsof base sheet resistivity (R_(sb)), the turn-on voltage can be expressedas a logarithmic function of base sheet resistance

V _(be) =−A In[R _(sb) ]+V _(o)  (3)

with

A=(kT/q)  (4)

and

V _(o) =E _(gb) /q−(kT/q)In[q ² μN _(c) N _(v) D _(n) /J _(c)]  (5)

[0068] where N_(c) and N_(v) are the effective density of states in theconduction and valence bands and μ is the majority carrier mobility inthe base layer.

[0069]FIG. 3 plots the turn-on voltage at J_(c)=1.78 A/cm² as a functionof base sheet resistivity for a number of InGaP/GaAs HBTs, GaAs/GaAsBJTs, and InGaP/GaInAsN DHBTs. The turn-on voltage of both theInGaP/GaAs HBTs and the GaAs/GaAs BJTs, which do not have any conductionband spike, qualitatively exhibit the same logarithmic dependence onbase sheet resistivity expected from equation (2). Quantitatively, thevariation of base-emitter voltage (V_(bc)) with base sheet resistivityis less severe than represented by equation (3) (A=0.0174 instead of0.0252 mV). However, this observed reduction in A is consistent with thequasiballistic transport through thin base GaAs bipolar devices.

[0070] Comparison with the characteristics of GaAs/GaAs BJTs leads tothe conclusion that the effective height of the conduction band spikeInGaP/GaAs HBTs can be zero, with the collector current exhibiting ideal(n=1) behavior. Thus, InGaP/GaAs HBTs can be engineered to haveessentially no conduction band spike. Similar results were found byprevious work for AlGaAs/GaAs HBTs. To further lower the turn-on voltagefor these devices for a fixed base sheet resistivity requires the use ofa base material with a lower energy gap but which still maintains theconduction band continuity. Ga_(1-x)In_(x)As_(1-y)N_(y) can be used toreduce E_(gb) while maintaining near lattice-matching conditions. Asseen in FIG. 3, the turn-on voltage of two sets of InGaP/GaInAsN DHBTsfollows a logarithmic dependence on base sheet resistivity indicatingthat the conduction band spike is about zero. In addition, the turn-onvoltage is shifted downward by 11.5 mV in one set and by 25.0 mV in theother set (dashed lines) from that observed for InGaP/GaAs HBTs andGaAs/GaAs BJTs.

[0071] The above experiment shows that the turn-on voltage of GaAs-basedHBTs can be reduced below that of GaAs BJTs by using a InGaP/GaInAsNDHBT structure. A low turn-on voltage is achieved through two key steps.The base-emitter interface is first optimized to suppress the conductionband spike by selecting base and emitter semiconductor materials inwhich the conduction bands are at about the same energy level. This issuccessfully done using InGaP or AlGaAs as the emitter material and GaAsas the base. A further reduction in turn-on voltage was thenaccomplished by lowering the band gap of the base layer. This wasachieved while still maintaining lattice matching throughout the entireHBT structure by adding both indium and nitrogen to the base layer. Withproper growth parameters, a two-fold increase in collector currentdensity was achieved without significantly sacrificing base doping orminority carrier lifetime (β=68 at R_(sb)=234 Ω/□). These resultsindicate that the use of a Ga_(1-x)In_(x)As_(1-y)N_(y) material providesa method for lowering the turn-on voltage in GaAs-based HBTs and DHBTs.Since incorporation of indium and nitrogen in GaAs lowers the band gapof the material, larger reductions in turn-on voltage within GaAs basedHBTs and DHBTs are expected as a larger percentage of indium andnitrogen is incorporated into the base if a high p-type dopingconcentration is maintained.

[0072] The energy-gap reduction in the GaInAsN base, assumed to beresponsible for the observed decrease in turn-on voltage, has beenconfirmed by low temperature (77° K.) photoluminescence. FIG. 4 comparesphotoluminescence spectra from an InGaP/GaInAsN DHBT and a conventionalInGaP/GaAs HBT. The base layer signal from the InGaP/GaAs HBT is at alower energy than the collector (1.455 eV vs. 1.507 eV) because ofband-gap-narrowing effects associated with high-doping-levels. The baselayer signal from the InGaP/GaInAsN DHBT which appears at 1.408 eV isreduced because of band-gap-narrowing effects and a reduction in thebase layer energy gap caused by incorporation of indium and nitrogen inthe base layer. In this comparison, the doping levels are comparable,suggesting the 47 meV reduction in the position of the base layer signalcan be equated to a reduction in the base layer energy gap in theGaInAsN base as compared with the energy gap of the GaAs base. Thisshift in photoluminescence signal correlates very well with the measured45 mV reduction in turn-on voltage. In the absence of a conduction bandspike, the turn-on voltage reduction can be directly related to thedecrease in base layer energy gap.

[0073] The DCRXD spectra shown in FIG. 5 illustrates the effect ofaddition of carbon dopants and indium to a GaAs semiconductor. FIG. 5shows the DCRXD spectra from both an InGaP/GaInAsN DHBT and a standardInGaP/GaAs HBT of comparable base thickness. In the InGaP/GaAs HBT, thebase layer is seen as a shoulder on the right hand side of the GaAssubstrate peak, approximately corresponding to a position of +90arcsecs, due to the tensile strain generated from the high carbon dopantconcentration of 4×10¹⁹ cm⁻³. With the addition of indium, the baselayer peak is at −425 arcsec in this particular InGaP/GaInAsN DHBTstructure. In general, the position of the peak associated with theGaInAsN base is a function of the indium, nitrogen, and carbonconcentrations. The addition of indium to GaAs adds a compressivestrain, while both carbon and nitrogen compensate with a tensile strain.

[0074] Maintaining high p-type doping levels as indium (and nitrogen)are added to carbon doped GaAs requires careful growth optimization. Arough estimate of the active doping level can be obtained from acombination of measured base sheet resistivity and base thicknessvalues. The base doping can also be confirmed by first selectivelyetching to the top of the base layer and then obtaining a Polaron C-Vprofile. FIG. 6 compares such Polaron C-V doping profiles from a GaAsbase layer and a GaInAsN base layer. In both case, doping levelsexceeded 3×10¹⁹ cm⁻³.

[0075]FIG. 7a shows an alternative structure for DHBTs having constantcomposition GaInAsN base layer (10) that employs transitional layers (20and 30) between the emitter/base and the collector/base junction. Inaddition, a lattice-match InGaP tunnel layer (40) is employed betweenthe transitional layer and the collector.

[0076] DHBTs with Compositionally-Graded Base Layers

[0077] All layers in DHBTs having a compositionally-graded base layercan be grown in a similar fashion as DHBTs having a base with a constantcomposition except the base layer as a graded band gap firm one junctionthrough the layer to another junction of the transistor. For example, acarbon-doped and bond gap-graded GaInAsN base layer can be grown overthe collector if neither a lattice-matched nor a transitional layer isused. Optionally, the carbon doped graded GaInAsN base layer can begrown over the transitional layer or over the lattice-matched layer if atransitional layer was not used. The base layer can be grown at atemperature below about 750° C. and typically is about 400 Å to about1500 Å thick. In one embodiment, the base layer is grown at atemperature of about 500° C. to about 600° C. The base layer can begrown using a gallium source, such as, for example, trimethylgallium ortriethylgallium, an arsenic source, such as arsine, tri(t-butyl)arsineor trimethylarsine, an indium source, such as trimethylindium, and anitrogen source, such as ammonia, dimethylhydrazine or t-butylamine. Alow molar ratio of the arsenic source to the gallium source ispreferred. Typically, the ratio molar ratio of the arsenic source to thegallium source is less than about 3.5. More preferably, the ratio isabout 2.0 to about 3.0. The levels of the nitrogen and indium sourcescan be adjusted to obtain a material in which the content of the GroupIII element is about 0.01% to about 20% indium and the content of theGroup V element is about 0.01% to about 20% nitrogen. In a specificembodiment, the content of the Group III element that is indium isvaried from about 10% to 20% at the base-collector junction to about0.01% to 5% at the base-emitter junction and the content of the Group Velement that is nitrogen essentially is constant at about 0.3%. Inanother embodiment, the nitrogen content of the base layer is aboutthree times lower than the indium content. As discussed above in regardto GaInAsN base layers having a constant composition, it is believedthat a GaInAsN layer having a high carbon dopant concentration, of about1.5×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³, can be achieved by using anexternal carbon source, such as a carbon tetrahalide, in addition to thegallium source. The external carbon source used can be, for example,carbon tetrabromide. Carbon tetrachloride is also an effective externalcarbon source.

[0078] Since organoindium compounds that are used as the indium sourcegas contribute a different amount of carbon dopant to the GaInAsN baselayer than the organogallium compounds that are used as the galliumsource gas, the carbon dopant source gas flow typically is adjustedduring the growth of the base layer so as to maintain a constant carbondoping concentration in the compositionally-graded GaInAsN base layer.In one embodiment, change in the carbon source gas flow over thecompositionally graded base layer is determined using the methoddescribed below.

[0079] Carbon and Trimethylindium Source Flow Rate Calibration Procedurefor Graded GaInAsN and/or Graded InGaAs Semiconductor Layers

[0080] At least two sets of calibration HBTs are prepared, in which eachset contains at least two members (DHBTs can be used instead of HBTs).The base layer thickness is ideally the same for all calibration HBTsformed but is not a requirement, and each HBTs has a constantcomposition, such as a constant composition of GaInAsN or GaInAs baselayer and a constant carbon dopant concentration throughout the layer.Each set is grown at a different Group III or Group V additive (such asindium for Group III or nitrogen for Group V) source gas flow rate thananother set so that the members of each set have a different gallium,indium, arsenic and nitrogen composition than that of members of adifferent set. By way of example, indium will be employed as theadditive which affects band gap gradation. Each member of a particularset is grown at a different external carbon source (e.g., carbontetrabromide, or carbon tetrachloride) flow rate so that each member ofa particular set has a different carbon dopant level. Thedoping*mobility product is determined for each member and graphedagainst the carbon source flow rate. The doping*mobility product variesproportionately with the carbon source gas flow rate for the members ofeach set. Doping*mobility product vs. carbon tetrabromide flow rate forfive sets of HBTs is graphed in FIG. 8. Alternatively, each set ofcalibration HBTs could be formed by maintaining a constant flow rate ofthe carbon source gas, such as carbon tetrabromide, and each separatesample in each set could be formed at a distinct Group III or Group Vadditive flow rate relative to the flow rate of the other sources gases.

[0081] The flow rate of carbon source gas versus indium source gasneeded to obtain a constant doping*mobility product is obtained bydrawing a line across the graph in FIG. 8 at a constant doping*mobilityproduct (e.g., a line parallel to the x-axis). Where this lineintersects with the straight lines of each set represents the externalcarbon source flow needed to obtain this doping*mobility product valuewhen the indium source gas flow is set at the flow rate for that set.The external carbon source gas flow rate versus indium source gas flowrate for one constant doping*mobility product value is graphed in FIG.9. Similar lines for different doping*mobility products can be graphedin the same way.

[0082] The collector current of each HBT is graphed as a function ofbase-emitter voltage (V_(be)) and the curve obtained is compared to agraph of an HBT that has a GaAs base layer but otherwise is identical tothe member in the set to which it is compared (e.g., has the same dopantconcentration, the same thickness of base, emitter and collector layers,ect.). The voltage difference between the curves at a particularcollector current is the change in the base emitter voltage,V_(be)(ΔV_(be)), attributed to the lower energy gap of the base layercaused by addition of indium and nitrogen during formation of the baselayer. FIG. 10 shows a plot of the collector current as a function ofV_(be) for an HBT having a GaInAsN base layer and an HBT having a baselayer of GaAs. The horizontal arrow drawn between the two curves is theΔV_(be). The ΔV_(be) for each member of all the sets is determined andplotted vs. carbon source gas flow. The ΔV_(be) vs. carbon tetrabromideflow for each member of the five sets of HBTs used to form the graph inFIG. 8 were plotted in FIG. 11. Note that the ΔV_(be) for the members ofa set span a range of ΔV_(be) for that set to which a straight line canbe fitted. These lines are then used to determin (interpolate) theΔV_(be) values for HBTs that could be grown using the same indium sourcegas flow rate of a particular set but with carbon source gas flow ratesthat are different from the other members of the set.

[0083] The ΔV_(be) for a constant doping*mobility product varieslinearly as a function of indium source gas flow rate, as can be seenwhen the interpolated ΔV_(be) for a constant doping*mobility product isplotted as a function of indium source gas flow rate. FIG. 12 shows thisplot for the five sets used in FIG. 11.

[0084] The graph shown in FIG. 12 is used to determine the indium sourcegas flow needed to obtain the desired ΔV_(be) at the base-emitter andbase/collector junctions. Once the indium source gas flow is determined,FIG. 9 is used to determine the carbon source gas flow needed at thatindium source gas flow to obtain the desired dopant*mobility product.The same procedure is followed to determine the desired indium sourcegas flow and carbon source gas flow at the base-collector junction tomaintain the desired constant dopant*mobility product in thecompositionally graded GaInAs or GaInAsN layer. The indium source gasflow and carbon source gas flow are varied linearly relative to thegallium and arsenic levels when the base layer is grown from thebase-collector junction to the base-emitter junction to the valuesdetermined for these source gases at these junctions to obtain alinearly graded base layer having a desired band gap grade.

EXAMPLE 2

[0085] All of the GaAs devices used in the following discussion wereMOCVD-grown, carbon-doped base layers in which the dopant concentrationvaried from about 3.0×10¹⁹ cm⁻³ to about 5.0×10¹⁹ cm⁻³ and a thicknesswhich varied from about 500 Å to about 1500 Å, resulting in a base sheetresistivity (R_(sb)) of between 100 W/□ and 650 W/□. Large area devices(L=75 mm×75 mm) were fabricated using a simple wet-etching process andtested in the common base configuration. Relatively small amounts ofindium (x˜1% to 6%) and nitrogen (y˜0.3%) were added incrementally toform two separate sets of InGaP/GaInAsN DHBTs. For each set, growth wasoptimized to maintain relatively high, uniform carbon dopant levels(>2.5×10¹⁹ cm⁻³), good mobility (˜85 cm²/V-s), and high dc current gain(>60 at R_(sb)˜300 W/square). The structure of a DHBT used in thefollowing experiments having a compositionally graded GaInAsN base layeris shown in FIG. 13. Alternative structures for DHBTs havingcompositionally garded base layers is shown in FIGS. 7b and 7 c. Thestructure of a DHBT having a constant composition GaInAsN base layerused in the following experiments for comparison is shown in FIG. 14.

[0086]FIG. 15 shows Gummel plots from a constant and a graded base DHBTwith comparable turn-on voltages and base sheet resistance. The neutralbase component of the base current is significantly lower in the gradedbase structure, which exhibits a peak dc current gain which can be overa factor of 2 higher than constant base structures. FIG. 16 compares dccurrent gain as a function of base sheet resistance from similarconstant and graded DHBT structures with varying thicknesses. Theincrease in gain-to-base-sheet-resistance ratio is readily apparent.While the gain-to-base-sheet-resistance ratio of DHBTs depends on thegrowth conditions utilized and the specific details of the overallstructure, a consistent 50% to 100% increase in dc current gain in DHBTswith a graded base layer over DHBTs with a constant base layer has beenobserved.

[0087]FIGS. 17 and 18 compare the Gummel plots and gain curves from agraded base structure to two constant base structures. The basecomposition of the first constant base structure corresponding to thecomposition of base layer of the graded base at base-emitter junction.The base composition of the second constant base structure correspondingto the composition of base layer of the graded base at base-emitterjunction. The turn-on voltage of the graded base structure isintermediate between the tow endpoint structures, but is weightedtowards the base-emitter endpoint. The dc current gain of the graded basstructure is between 50% and 95% higher than the endpoint structures,indicating most of the increase in dc current gain results from anincreased electron velocity.

[0088] On-wafer FF testing was performed using an HP8510C parametricanalyzer on 2 finger, 4 μm×4 μm emitter area devices. Pad parasitic werede-embedded using open and short structures, and the current gain cutofffrequency (f_(t)) was extrapolated using a −20 dB/dicade slope of thesmall signal current gain (H21). FIG. 19 summarizes the f_(t) dependencewith collector current density (J_(c)) on both structures. FIG. 20illustrates the small signal gain versus frequency at one particularbias point. As J_(c) increases and the base transit time (t_(b)) beginsto play a limiting role in the total transit time, the f_(t) of thegraded base structure becomes notably larger than the constantcomposition structure, despite the greater base thickness of the gradedbase structure (constant base layer is 60 nm thick whereas graded baselayer is 80 nm thick). The peak f_(t) of the 60 nm constant compositionGaInAsN base is 53 GHz, while the 80 nm compositionally graded GaInAsNbase has a peak f_(t) of 60 GHz. Thus, the current gain cutoff frequencyis increased by 13%. To better compare the RF results of DHBTs having aconstant and a graded GaInAsN base layer to one another and toconventional GaAs HBTs, the f_(t) values form FIG. 19 were plotted as afunction of the zero-input-current breakdown voltage (BV_(ceo)) that canbe applied to the transistor. This plot was compared to the peak or nearpeak f_(t) values of conventional GaAs HBTs quoted in the literature. Afairly wide distribution in the f_(t) values of the conventional GaAsHBTs was expected, as this data was compiled from many groups usingdifferent epitaxial structures, device sizes, and test conditions, andis only meant to give a sense of current industry standards. TheBV_(ceo) most often had to be estimated from quoted collector thicknessassuming the relations between the collector thickness (X_(c)), BV_(cbo)and BV_(ceo) shown in FIG. 21. Also shown in FIG. 21 are three simplecalculations of the expected dependence of f_(t) on BV_(ceo) assumingthe transit time through the space charge layer of collector (τ_(sclc))is simply related to X_(c) via the electron saturation (drift) velocity(v_(s)). In the baseline calculation, the τ_(b) is assumed to be 1.115ps, as expected from Monte-Carlo calculations for a 1000 Å GaAs baselayer, and the sum of the remaining emitter and collector transit times(τ_(e)+τ_(c)) was taken as 0.95 ps.

[0089] Examination of FIG. 21 indicates that while the f_(t) of theconstant composition GaInAsN is not entirely outside the range expectedfor conventional GaAs-based HBTs, it is clearly on the low end of thedistribution. The graded base structure is notably improve. The secondcalculation (baseline with τ_(b) reduced by ⅔) suggests the base transittime was decreased by approximately 50% relative to the constantcomposition structure. This indicate that a 2× increase in velocity ofcarriers was achieved in the graded base layer as compared to theconstant composition base layer since a 2× increase in velocity combinedwith a 33% increase in base thickness is expected to lead to a reductionin τ_(b) of ½×{fraction (4/3)}=⅔. The third calculation (baseline withτ_(b) reduced by ⅓ and (τ_(e)+τ_(c)) by ½) approximates situations inwhich thin and/or graded base structures are employed along withimproved device layout and size (to minimize τ_(b), τ_(e) and τ_(c)).

EQUIVALENTS

[0090] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A heterojunction bipolar transistor, comprising:a) an n-doped collector; b) a base comprising a III-v material formedover the collector, wherein the III-V material includes indium andnitrogen, and wherein the base is doped with carbon at a concentrationof about 1.5×10¹⁹ cm⁻³ to about 7.0×10¹⁹ cm⁻³; and c) an n-doped emitterformed over the base.
 2. The transistor of claim 1, wherein the basecomprises the elements gallium, indium, arsenic, and nitrogen.
 3. Thetransistor of claim 2, wherein the collector is GaAs and the emitter isInGaP, AlInGaP, or AlGaAs and the transistor is a double heterojunctionbipolar transistor.
 4. The transistor of claim 2, wherein the base layerband gap is lower at a surface of the base layer that is in contact withthe collector than the band gap at a surface of the base layer that isin contact with the emitter by an amount in a range of between about 20meV and about 120 meV.
 5. The transistor of claim 4, wherein the bandgap of the base layer is linearly graded from the surface of the baselayer in contact with the collector to the surface of the base layer incontact with the emitter.
 6. The base layer of claim 5, wherein theaverage band gap reduction in the graded base layer is in a range ofbetween about 20 meV and about 300 meV less than the band gap of GaAs.7. The base layer of claim 6, wherein the average band gap reduction inthe graded base layer is in a range of between about 80 meV and about300 meV less than the band gap of GaAs.
 8. The base layer of claim 6,wherein the average band gap reduction in the graded base layer is in arange of between about 20 meV and about 200 meV less than the band gapof GaAs.
 9. The transistor of claim 3, wherein the base layer comprisesa layer of the formula Ga_(1-x)In_(x)As_(1-y)N_(y), wherein x and y areeach, independently, about 1.0×10⁻⁴ to about 2.0×10⁻¹.
 10. Thetransistor of claim 9, wherein x is about equal to 3y.
 11. Thetransistor of claim 9, wherein x has a value in a range of about 0.2 toabout 0.02 at the collector and is graded to a value in a range of about0.1 to about zero at the emitter, provided that x is larger at thecollector than at the emitter.
 12. The transistor of claim 11, wherein xis about 0.06 at the collector and about 0.01 at the emitter.
 13. Thetransistor of claim 10, wherein the base layer is about 400 Å to about1500 Å thick and has a sheet resistivity of about 100 Ω/square to about400 Ω/square.
 14. The transistor of claim 13, wherein the n-dopant inthe emitter is present in a concentration range of between about3.5×10¹⁷ cm⁻³ and about 4.5×10¹⁷ cm⁻³ and the concentration of then-dopant in the collector is in a range of between about 9×10¹⁵ cm⁻³ andabout 2×10¹⁶ cm⁻³.
 15. The transistor of claim 14, wherein the emitterand the collector are doped with silicon.
 16. The transistor of claim15, wherein the emitter is about 500 Å to about 750 Å thick, and thecollector is about 3500 Å to about 4500 Å thick.
 17. The transistor ofclaim 16, further comprising a first transitional layer disposed betweenthe base and the collector, said first transitional layer having a firstsurface contiguous with a first surface of the base, and wherein thefirst transitional layer includes an n-doped material selected from thegroup consisting of GaAs, InGaAs and InGaAsN.
 18. The transistor ofclaim 16, further comprising a second transitional layer having a firstsurface contiguous with a first surface of the emitter and a secondsurface contiguous with a second surface of the base, wherein the secondtransitional layer includes an n-doped material selected from the groupconsisting of GaAs, InGaAs and InGaAsN.
 19. The transistor of claim 16,further comprising a lattice-matched layer having a first surfacecontiguous with a first surface of the collector and a second surfacecontiguous with a second surface of the first transitional layer,wherein the lattice matched layer is a wide-band-gap material.
 20. Thetransistor of claim 19, wherein the lattice-matched layer is selectedfrom the group consisting of InGaP, AlInGaP and AlGaAs.
 21. Thetransistor of claim 18, wherein the first and the second transitionallayers are about 40 Å to about 60 Å thick.
 22. The transistor of claim19, wherein the first and the second transitional layers are about 40 Åto about 60 Å thick and the lattice matched layer is about 150 Å toabout 250 Å thick.
 23. A method of fabricating a heterojunction bipolartransistor, comprising the steps of: a) growing a base layer comprisinggallium, indium, arsenic and nitrogen over an n-doped GaAs collectorlayer from a gallium, indium, arsenic, and nitrogen source, wherein thebase layer is p-doped with carbon from an external carbon source; and b)growing an n-doped emitter layer over the base layer.
 24. The method ofclaim 23, wherein the external carbon source is carbon tetrabromide orcarbon tetrachloride.
 25. The method of claim 24, wherein the galliumsource is selected from trimethylgallium and triethylgallium.
 26. Themethod of claim 25, wherein the nitrogen source is ammonia,dimethylhydrazine or tetiarybutylamine.
 27. The method of claim 26,wherein the ratio of the arsenic source to the gallium source is about2.0 to about 3.5.
 28. The method of claim 27, wherein the base is grownat a temperature of less than about 750° C.
 29. The method of claim 28,wherein the base is grown at a temperature of about 500° C. to about600° C.
 30. The method of claim 28, wherein the base layer comprises alayer of the formula Ga_(1-x)In_(x)As_(1-y)N_(y), wherein x and y areeach, independently, about 1.0×10⁻⁴ to about 2.0×10⁻¹.
 31. The method ofclaim 30, wherein x is about equal to 3y.
 32. The method of claim 30,wherein the collector includes GaAs and the emitter includes a materialselected from the group consisting of InGaP, AlInGaP, and AlGaAs, andwherein the transistor is a double heterojunction bipolar transistor.33. The method of claim 30, further comprising the step of growing ann-doped first transitional layer over the collector layer prior togrowing the base layer, and wherein the base layer is grown over then-doped first transition layer, and wherein the first transitional layerhas a graded band gap or a band gap that is smaller than the band gap ofthe collector.
 34. The method of claim 33, wherein the firsttransitional layer is selected from the group consisting of GaAs,InGaAs, and InGaAsN.
 35. The method of claim 34, further comprising thestep of growing a second transitional layer over the base prior togrowing the n-doped emitter layer, wherein the second transitional layerhas a first surface contiguous with a surface of a first surface of thebase and a second surface contiguous with a surface of the emitter, andwherein the second transitional layer has a doping concentration atleast one order of magnitude less than the doping concentration of theemitter.
 36. The method of claim 35, wherein the second transitionallayer is selected from the group consisting of GaAs, InGaAs, andInGaAsN.
 37. The method of claim 36, wherein the first transitionallayer, the second transitional layer, or both the first and the secondtransitional layer formed have a doping spike.
 38. The method of claim36, further comprising the step of growing a latticed matched layer overthe collector prior to growing the n-doped first transitional layer,wherein the lattice matched layer has a first surface contiguous with afirst surface of the collector and a second surface contiguous with asecond surface of the first transitional layer.
 39. The method of claim38, wherein the lattice matched layer includes InGaP.
 40. A materialcomprising gallium, indium, arsenic, and nitrogen, wherein the materialis doped with carbon at a concentration of about 1.5×10¹⁹ cm ⁻³ to about7.0×10¹⁹ cm⁻³.
 41. The material of claim 40, wherein the composition ofthe material can be represented by the formulaGa_(1-x)In_(x)As_(1-y)N_(y), wherein x and y are each, independently, ina range of between about 1.0×10⁻⁴ and about 2.0×10⁻¹.
 42. The materialof claim 41, wherein x is about equal to 3y.
 43. The material of claim42, wherein x and 3y are about 0.01.
 44. The material of claim 43,wherein the carbon concentration is at least about 3.0×10¹⁹ cm⁻³.
 45. Amaterial comprising gallium, indium, arsenic, and nitrogen, wherein thecomposition of the material is represented by the formulaGa_(1-x)In_(x)As_(1-y)N_(y), wherein x and y are each, independently,linearly graded from a larger value at a first surface of the materialto a smaller value at a second surface of the material.
 46. The materialof claim 45, wherein the material is doped with carbon.
 47. The materialof claim 46, wherein x is linearly graded from about 0.01 to about 0.06from a second surface of the material to a first surface of thematerial.
 48. A material comprising gallium, indium, arsenic, andnitrogen, wherein the composition of the material is represented by theformula Ga_(1-x)In_(x)As_(1-y)N_(y), wherein x is linearly graded from alarger value at a first surface of the material to a smaller value at asecond surface of the material and y remains essentially constantthroughout the material.
 49. The material of claim 48, wherein thematerial is doped with carbon.
 50. The material of claim 49, wherein xis linearly graded from about 0.01 to about 0.06 and y is about 0.001from a second surface of the material to a first surface of thematerial.
 51. A method of forming a graded semiconductor layer having anessentially linear grade of band gap and an essentially constantdoping-mobility product, from a first surface through the layer to asecond surface, comprising the steps of: a) comparing thedoping-mobility product of calibration layers, each of which is formedat a distinct flow rate of one of either an organometallic compounddepositing an atom from Group III or V of the Periodic Table, or of acarbon tetrahalide compound depositing carbon, whereby the relativeorganometallic compound and carbon tetrahalide flow rates required toform an essentially constant doping-mobility product are determined; andb) flowing the organometallic and carbon tetrahalide compounds over asurface at said relative rates to form an essentially constantdoping-mobility product, said flow rates changing during deposition tothereby form an essentially linear grade of band gap through the gradedsemiconductor layer.
 52. The method of claim 51, further including thestep of depositing the graded layer on a second semiconductor layerduring fabrication of a junction device.
 53. The method of claim 52,wherein the second semiconductor layer is a collector layer.
 54. Themethod of claim 52, wherein the second semiconductor layer is an emitterlayer.
 55. The method of claim 51, wherein the graded semiconductorlayer includes gallium, indium and arsenic, and wherein theorganometallic compound that determines the rate of deposit of thecarbon tetrahalide to form an essentially constant doping-mobilityproduct includes an organo-indium compound.
 56. The method of claim 55,wherein the carbon tetrahalide is CBr₄.
 57. The method of claim 56,wherein the organometallic compound further includes a nitrogen sourcegas.
 58. The method of claim 57, wherein the second semiconductor layerin which the graded semiconductor layer is deposited includes GaAs. 59.The method of claim 58, further including the step of depositing a thirdsemiconductor layer on the base layer.
 60. The method of claim 59,wherein the third semiconductor layer is InGaP.
 61. The method of claim58, wherein the doping-mobility product for each calibration layer isrelated to a bandgap, whereby bandgaps at the first and second surfacesof a graded layer, in combination with a doping-mobility product, willbe calibrated to relative rates of organometallic and carbon tetrahalideflow rates required for deposition of said graded semiconductor layer.62. The method of claim 61, wherein said bandgaps are calibrated asbase-emitter voltages of junction devices employing said calibrationlayers as base layers, relative to GaAs.
 63. The method of claim 62,wherein the graded semiconductor base layer formed is a base layer in aheterojunction bipolar transistor.
 64. The method of claim 63, whereinthe flow rates of the organometallic and carbon tetrahalide cause thebandgap of the resultant graded base layer to decrease from abase-emitter junction to a base-collector junction of saidheterojunction bipolar transistor.
 65. A semiconductor material made bythe method of claim 51.